Light polarization employing magnetically oriented ferrite suspensions

ABSTRACT

Light polarization is provided by disposing finely divided ferrite particles in an inert light-transmitting medium on a transparent support to form a suspension of the particles in the medium, applying a magnetic field to the suspension to orient the particles into line patterns which polarize visible light transmitted through the suspension and detecting the polarized light by suitable means.

Goldberg et a1.

LIGHT POLARIZATION EMPLOYING MAGNETICALLY ORIENTED FERRITE SUSPENSIONS Inventors: Paul Goldberg, West Newton; Pieter J. van Heel-den, Concord, both of Appl. No.: 469,373

Related US. Application Data Continuationin-part of Ser. No. 270,526, July 10, 1972, abandoned, which is a continuation-in-part of Ser. No. 86,997, Nov. 4, 1970, abandoned.

427/47; 427/128; 428/900 Int. Cl. G02F 1/09 Field of Search 117/234-240',

References Cited UNITED STATES PATENTS 12/1960 Blume 18/475 1 Dec. 23, 1975 2,971,916 2/1961 Schleicher et a1. 117/238 X 3,066,355 12/1962 Schloemann et al. 252/6254 X 3,067,140 12/1962 252/6254 X 3,189,550 6/1965 Malinofsky 252/625 3,221,315 11/1965 Brown et a1. 117/235 X 3,320,523 5/1967 Trimble 350/151 UX 3,421,002 1/1969 Berger 117/238 X 3,683,382 8/1972 Ballinger 117/238 X OTHER PUBLICATIONS Magnetic Fluids, Rosensweig, pp. 48-54 and 56, International Science & Tech., 7-66.

Primary Examiner-Bamard D. Pianalto Attorney, Agent, or Firm-Mart C. Matthews; Philip G. Kiely [57] ABSTRACT Light polarization is provided by disposing finely divided ferrite particles in an inert light-transmitting medium on a transparent support to form a suspension of the particles in the medium, applying a magnetic field to the suspension to orient the particles into line patterns which polarize visible light transmitted through the suspension and detecting the polarized light by suitable means.

9 Claims, No Drawings LIGHT POLARIZATION EMPLOYING MAGNETICALLY ORIENTED FERRITE SUSPENSIONS CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of copending application Ser. No. 270,526 filed July 10, 1972 now abandoned which in turn was a continuation-inpart of application Ser. No. 86,997 filed Nov. 4, 1970 and now abandoned.

BACKGROUND OF THE INVENTION wherein one axis is relatively long compared to another, in a fluid medium and then subjecting them to an electric or magnetic field. After the axes have been aligned in the field, it is possible to transmit one component of incident radiation through the medium between the crystals in the direction of the long axis while the suspended bodies absorb a second component to thus polarize the radiation. Usually, such suspended particles are dichroic (i.e., are light polarizers in their own right) and transparent in the fluid medium. In these cases, the transmitted polarized light has an intensity relative to the incident light which depends on the strength of the field. For an example of light polarizing devices of the above-described type, see Land U.S. Pat. No. 1,955,923.

Stable suspensions of very fine particles of solid ferromagnetic material, e.g., ferrites, in inert liquids are known and commercially available. These suspensions, known as ferrofluids, have been subjected to magnetic fields in the past and a wide variety of interesting physical phenomena observed. In contrast to the suspended crystals of the aforementioned patent, these ferrite particles may be considered substantially spherical (i.e., symmetrical) in shape and optically neutral. See, for example, the article entitled Magnetic Fluids" by R. E. Rosensweig, International Science and Technology, 56 (July 1966), pages 48-54. Prior to the present invention, the ability of these ferrofluids under the influence of a magnetic field to polarize light has not been known or appreciated by the art. It is to this discovery which the present invention is directed.

SUMMARY OF THE INVENTION In accordance with this invention, finely divided ferrite particles, e.g., having a particle size of about 0.02 micron in diameter, are suspended in a light-transmitting inert medium, preferably water, on a transparent support. A magnetic field is applied to the ferrite suspension whereupon the ferrite particles orient themselves in the medium to form elongated, line-shaped agglomerates which polarize visible light passing through the magnetized suspension. The polarization of the transmitted light is readily detected by suitable detection means, e.g., by employing a polarimeter or crossed" polarizers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Ferrites are compounds ofiron, e.g., NaFeO,, which are known and have been of significant recent interest in the electronics art because of their insulating properties. As previously indicated, suspensions of finely divided ferrites in inert (i.e., non-optically active) liquids such as water, kerosene or silicone oil are commercially available and readily obtained. Typically, a surfactant is also included in the suspension to prevent clumping of particles and to permit disposition of more uniformly distributed particles in the suspending medium.

In the practice of this invention, such a ferrite sus pension may be placed on a suitable transparent support, for example, glass or a polymeric material such as polymethyl methacrylate, exposed to a source of visible light and subjected to a magnetic field, for example, by disposing the supported suspension between the poles of a magnet. Since polarized light is not apparent to the unaided eye, the polarization of the light which is transmitted through the suspension while under the influence of the magnetic field may be observed by employing conventional light polarization detection means such as a polarimeter, polariscope, crossedpolarizers, etc. The polarization of the present invention is such that the electrical vector of the light wave parallel to the lines of force of the magnetic field is absorbed (this vector is that which is reflected from a water surface at an angle of about 50) while the perpendicular vector is transmitted.

The number of ferrite particles per unit area, i.e., the thickness of a layer comprising the ferrite suspension, is not critical. Polarization in accordance with this invention may be achieved substantially as efficiently through a relatively thin layer of the suspension as through a relatively thick layer. Typically, suspensions having a concentration of ferrites from about 0.02 g per cc to about 0.50g per cc may be employed in the present invention.

In contrast with the prior art polarizing suspensions previously mentioned, the strength of the magnetic field is not critical to the light polarization of this invention. It is only necessary that sufficient magnetic force be applied to orient the particles into the abovedescribed line-shaped patterns (i.e., the saturation magnetization). Increasing the force of the magnetic field in excess of the saturation magnetization of the particles does not change the efficiency of the polarizer. Polarization does diminish gradually, however, as the distance at which the suspension is placed from the magnetic field increases. The time required for the ferrite suspension to acquire its polarizing properties under the influence of the magnetic field is essentially equal to the rise time of the magnetic field. Thus, the time of formation of the polarizer of this invention is measured in microseconds, e.g., less than about microseconds.

It is to be understood that the magnetic force applied in the practice of the invention need not be stationary. For example, a rotating polarizer could be formed by arranging for the rotation of the magnetic field.

A variety of suspending media may be employed, provided the media are not optically active. As examples of suitable suspending media, mention may be made of water, kerosene, silcones, and polymerizable monomers. It is possible to obtain a polarizer which 3 retains its polarizing properties when removed from the magnetic field by selecting the appropriate medium and, while under the influence of the magnetic field, setting the medium to a solid or rigid form. For examthe density, i.e., independent of the number of elements per square centimeter.

TABLE 1 pie, the medium may comprise a polymerizable mono- Wale! Base f s p mer which y be polymerized while in the magntic concentrat izii ifi gr/gt: Pii cij sfe 0.02 micron field to freeze the ferrites in the oriented attitude in the rm i. D solid polymeric medium. 050 L42 A particularly preferred suspending medium is water 0.87 0.60 1.44 since stronger polarization of light is achieved with 3 g 1'22 water than with other media. When water is employed, Concentration 0 1 y p l size' 002 micron there is also exhibited a wavelength independent scattering of light which under favorable conditions of 8: 3% if}? illumination, shows up as a brilliant red color when Concentration 0.05 gJcc. Particle size 0.02 micron observed at certain angles. A permanent polarizer may g2? g3? l 50 be prepared when water is the suspending medium 1 2 1 simply by freezing the suspension. It is interesting to Concen g-lc Pa t c e si micron note that this freezing results in a large decrease in the 0 38 8;: l 38 degree of polarization over that shown by water, and 0:4! 0:31 1:33 r a h th t Shown b other li id di h as Concentration 0.] gJcc. Particle size 0.02 micron kerosene and silicone oil. 31%,, 8%?

To investigate the physical changes in the ferrite 0.40 0.29 L40 suspension when subjected to a magnetic field to form 3g;5 g; gggfijg; 0 02 micron a light polarizer in accordance with this invention, a 2,01 wax imprint was made of the surface of a layer compris- 2J4 118 ing a water base suspension of ferrite particles, about 5% $122 0.02 micron in diameter and at a concentration of about 0.1 g per cc, frozen while under the influence of a magnetic field of about 2000 gauss. A carbon replica TABLE 2 was made of the wax imprint and the pattern of the particles recorded employing an electron microscope Kerosene Base Ferrite Suspension at 100,000 X magnification. The photomicrographs 233 3; 131523 0 02 micron thus obtained showed regular line-shaped agglomerates Dm D of ferrite particles about 0.4 micron apart. ln addition, 0 68 0 64 1 07 similar layers were also dried while in the magnetic field and inspected in an optical microscope with a 100 0-83 1.06 X oil immersion objective. Fine regular line patterns could be observed with a minimum line distance of Observations it may be com A comparison of the data set forth in Tables I and 2 cluded that the magnetic field causes the ferrite parti- Supports the W ,that the PQIMIZPUOH Prov'ded cles to assemble in line patterns in the liquid med-mm by the present invention is substantially independentof very Similar to the behavior of iron filings on a glass the thickness of the layer, that IS, the numberof ferrite plate, and that the very small distance between these Pamcles Per centmeter h as mdicated line patterns results in high absorption of the electric as the susPendmg medlum prov 1de,s a vector which is parallel to the lines. Furthermore, it is Polanzer 3 m hlgber degree of Polanzahon believed that each individual line acts as a separate other supendmg polarizing absorber since the polarizers of the present Smce certam f' may be f m the above invention, being colorless with respect to transmitted P" andprocess W h a from the Scope light act independently of wavelength in the visibk, of the invention herein involved, it is intended that all region of the spectrum. In contrast, the polarizing acmatter f m the l f flp be tion of known optical conducting gratings falls off P" 'llus tratlv e and not m a hmmng Sensesteeply at the wavelength which is equal to the distance claimed I between the lines in the grating. Light Polarizauon apparatus compnsmgi ln Tables I and 2 below are recorded respective a Source of Visible g quantitative data of the degree of polarization of transa light polarizer exposed to said visible light and mitted light from water base and kerosene base polarizcomprising finely divided ferrite Particles ers of the present invention. For each polarizer, several pended in an inert light-transmitting medium on a density readings were taken at different locations on transparent pp Said ferrite Particles being h polarizer Th D l (I /I h r 1,, i th magnetically oriented in said medium into lineintensity of polarized light illuminating the polarizer shaped patterns which polarize said visible light and I, is the intensity of the transmitted light at maxitransmitted through said suspension; and mum density. D log (I /I where I, is the transmeans for detecting said transmitted polarized light. mitted light at minimum density. The dichroic ratio 2. Apparatus as defined in claim 1 wherein said medium is liquid.

3. Apparatus as defined in claim 1 wherein said medium is water.

(D /D is a good measure of the degree of polarization since if the light is polarized by a number of individual elements or particles, D is independent of 4. Apparatus as defined in claim 1 wherein the parti- F through a Suspension form a light polarcle size of said ferrite particles is about 0.02 micron in and diameter detecting said transmitted polarized light by observing said light through conventional polarized light 5. Apparatus as defined in claim 1 wherein the con- 5 detection means centration of said ferrite particles in said suspension is 7 A method as d fi d in claim 6 wherein said liquid about 0.02 to 0.50 gram per cc. medium is water.

6. A method of light polarization which comprises: 8. A method as defined in claim 6 wherein said liquid exposing to a source f visible light a suspension f medium is set to a rigid form while under the influence finely divided ferrite particles in an inert liquid f. therepyapolamerwhlch medium on a transparent pp retains HS light polanzing properties when removed from said magnetic field.

sub ecting said suspension to a magnetic fie d 9. A method as defined in claim 8 wherein said settaent said particles in said medium into line-shaped bl di i a l i bl monomer, patterns which polarize said visible light transmitl5 

1. LIGHT POLARIZATION APPARATUS COMPRISING: A SOURCE OF VISIBLE LIGHT, A LIGHT POLARIZER EXPOSED TO SAID VISIBLE LIGHT AND COMPRISING FINELY DIVIDED FERRITE PARTICLE SUSPENDED IN AN INERT LIGHTTRANSMITTING MEDIUM ON A TRANSPARENT SUPPORT, SAID FERRITE PARTICLES BEING MAGNETICALLY ORIENTED IN SAID MEDIUM INTO LINE-SHAPED PATTERNS WHICH POLARIZE SAID VISIBLE LIGHT TRANSMITTED THROUGH SAID SUSPENSION, AND MEANS FOR DETECTING SAID TRANSMITTED POLARIZED LIGHT.
 2. Apparatus as defined in claim 1 wherein said medium is liquid.
 3. Apparatus as defined in claim 1 wherein said medium is water.
 4. Apparatus as defined in claim 1 wherein the particle size of said ferrite particles is about 0.02 micron in diameter.
 5. Apparatus as defined in claim 1 wherein the concentration of said ferrite particles in said suspension is about 0.02 to 0.50 gram per cc.
 6. A METHOD OF LIGHT POLARIZATION WHICH COMPRISES: EXPOSING TO A SOURCE OF VISIBLE LIGHT, A SUSPENSION OF FINELY DIVIDED FERRITE PARTICLES IN AN INERT LIQUID MEDIUM ON A TRANSPARENT SUPPORT; SUBJECTING SAID SUSPENSION TO A MAGNETIC FIELD TO ORIENT SAID PARTICLES IN SAID MEDIUM INTO LINE-SHAPED PATTERNS WHICH POLARIZE SAID VISIBLE LIGHT TRANSMITTED THROUGH SAID SUSPENSION TO FORM A LIGHT POLARIZER; AND DETECTING SAID TRANSMITTED POLARIZED LIGHT BY OBSERVING SAID LIGHT THROUGH CONVENTIONAL POLARIZED LIGHT DETECTION MEANS.
 7. A method as defined in claim 6 wherein said liquid medium is water.
 8. A method as defined in claim 6 wherein said liquid medium is set to a rigid form while under the influence of said magnetic field to form thereby a polarizer which retains its light polarizing properties when removed from said magnetic field.
 9. A method as defined in claim 8 wherein said settable medium is a polymerizable monomer. 